1. Field of the Invention
The present invention relates to methods for removing and decomposing halogenated hydrocarbon fumigation agents such as methyl bromide. More particularly, the invention provides methods using a substrate such as activated carbon to capture methyl bromide from fumigation gas streams and using thiosulfate to detoxify methyl bromide and thereby reduce or eliminate methyl bromide emissions into the atmosphere during commodity and structure fumigation.
2. Description of the Art
Methyl bromide (bromomethane, CH.sub.3 Br, CAS No. 74-83-9) is an important fumigant for treating agricultural commodities and structures. It is the most widely used fumigant for pest control in perishable produces such as, fresh fruits and vegetables, and cut flowers, endurable produces such as, cereal grains, dry fruits and nuts, and timber, and structures such as, mills, aircraft, and ships. Fumigation with methyl bromide is mandatory for import and export of many agricultural products in international trade. The worldwide use of methyl bromide for fumigating commodities and structures reached 1.8.times.10.sup.7 kg in 1992. It is estimated that 80-95% of the applied methyl bromide is emitted into the air as a result of intentional discharge of methyl bromide waste gases at the end of fumigation and leakage during fumigation.
Although methyl bromide is an effective fumigant, there is serious concern about its use because (1) in the air, it is acutely toxic, and (2) emission of methyl bromide during commodity and soil fumigation are reportedly contributing to stratospheric ozone depletion. The use of methyl bromide has thus been proposed to be discontinued in the United States by 2001, and in the other countries at 2010. However, as there are no effective alternatives, retaining methyl bromide for postharvest and structural fumigation is of great economic importance. Extension of methyl bromide usage may be permitted only if its emissions are sufficiently reduced. As commodity or structural fumigations are always carried out in closed environments, recovering unreacted methyl bromide is feasible. The fumigated products absorb little methyl bromide, leaving most of the applied chemical available for recovery.
Methods have been devised to recycle and recover methyl bromide, for example, by adsorption of methyl bromide on a molecular sieve and then desorption with hot gas (see U.S. Pat. No. 5,505,908).
Various methods have been proposed to recover and decompose methyl bromide from waste fumigation gases. These include passing the waste gas over metal catalysts, such as the oxides of iron, manganese, cobalt, and copper, at high temperatures (100-400.degree. C.), and further reacting with sodium or calcium hydroxide to neutralize the produced hydrogen bromide (Japanese Patent No.73,01,297; Japanese Patent No. 77,04,470); bubbling the waste gas through ethanolamine in the presence of sodium or calcium hydroxide, and further separating the reaction products through solvent extraction and ion-exchangers (Japanese Patent No. 74,127,862); and incinerating the waste gas at high temperature (600-1000.degree. C.), and then treating with sodium hydroxide solution to neutralize the produce hydrogen bromide (Japanese Patent No. 74,61,329; Japanese Patent No. 75,159,462; French Patent No. 231,309). Although these methods have proved successful at experimental scales, they have one or more of the following disadvantages: (1) use of high temperatures, which requires a good containment to prevent methyl bromide from leaking; (2) multiple procedures which require the construction of complex systems and lead to high costs; (3) unsafe or toxic end products such as hydrogen bromide or ethanolamine-hydrogen bromide salt and ethanolamine quaternary ammonium bromide; (4) reactive chemicals such as strong bases; and (5) stringent reaction conditions. These deficiencies have prevented the implementation of any one of these methods on any practical scale.
Various methods have been proposed to remove methyl bromide using activated carbon. U.S. Pat. No. 4,018,704 describes desorption of methyl bromide contained on active carbon using vapor (60-130.degree. C.) of an organic solvent that is inert to methyl bromide. Other methods include drawing methyl bromide waste gases from a fumigation room through an adsorption bed of activated carbon, and discharging the `filtered` air directly into the atmosphere. After methyl bromide is adsorbed on carbon, heated, moist nitrogen gas (130-250.degree. C.) (Japanese Patent No. 74,22,683), or hot methanol vapor (80.degree. C.) (German Patent No. 2,550,504), or hot air (280.degree. C.) (Japanese Patent No. 80,64,827 is used to desorb the methyl bromide. The desorbed methyl bromide is further incinerated at high temperatures (600.degree. C.) in a special furnace, and the hydrogen bromide produced from combustion is finally scrubbed in sodium hydroxide solution to neutralize it. Although some of these methods are successful on experimental and small production scales, none of them has found wide application. The lack of application may be attributed to the following disadvantages: (1) the need to precisely control the time and flow rate in order to eliminate any significant breakthrough of methyl bromide through the adsorption bed; (2) the need for multiple steps and stringent conditions to detoxify methyl bromide-loaded carbon, which gives the necessity for constructing complex detoxification facilities to fulfill these steps and performing the detoxification process off site separately from the removal process; (3) the difficulties and cost involved in transporting the toxic carbon with permissions from environmental regulating agencies to a location where a detoxification facility is available; and (4) high time consumption and cost to accomplish detoxification of methyl bromide and to regenerate the activated carbon.
What is needed is an inexpensive, safe, and on-site executable method to remove and detoxify methyl bromide from fumigation discharge gases.